Li Xun-Lu, Wang Tian, Yuan Yifei, Yue Xin-Yang, Wang Qin-Chao, Wang Jun-Yang, Zhong Jun, Lin Ruo-Qian, Yao Yuan, Wu Xiao-Jing, Yu Xi-Qian, Fu Zheng-Wen, Xia Yong-Yao, Yang Xiao-Qing, Liu Tongchao, Amine Khalil, Shadike Zulipiya, Zhou Yong-Ning, Lu Jun
Department of Materials Science, Fudan University, Shanghai, 200433, P. R. China.
Department of Chemistry, Fudan University, Shanghai, 200433, P. R. China.
Adv Mater. 2021 Apr;33(13):e2008194. doi: 10.1002/adma.202008194. Epub 2021 Mar 1.
Oxygen-redox of layer-structured metal-oxide cathodes has drawn great attention as an effective approach to break through the bottleneck of their capacity limit. However, reversible oxygen-redox can only be obtained in the high-voltage region (usually over 3.5 V) in current metal-oxide cathodes. Here, we realize reversible oxygen-redox in a wide voltage range of 1.5-4.5 V in a P2-layered Na Mg [Fe Mn □ ]O cathode material, where intrinsic vacancies are located in transition-metal (TM) sites and Mg-ions are located in Na sites. Mg-ions in the Na layer serve as "pillars" to stabilize the layered structure during electrochemical cycling, especially in the high-voltage region. Intrinsic vacancies in the TM layer create the local configurations of "□-O-□", "Na-O-□" and "Mg-O-□" to trigger oxygen-redox in the whole voltage range of charge-discharge. Time-resolved techniques demonstrate that the P2 phase is well maintained in a wide potential window range of 1.5-4.5 V even at 10 C. It is revealed that charge compensation from Mn- and O-ions contributes to the whole voltage range of 1.5-4.5 V, while the redox of Fe-ions only contributes to the high-voltage region of 3.0-4.5 V. The orphaned electrons in the nonbonding 2p orbitals of O that point toward TM-vacancy sites are responsible for reversible oxygen-redox, and Mg-ions in Na sites suppress oxygen release effectively.
层状结构金属氧化物阴极的氧氧化还原作为突破其容量限制瓶颈的有效方法已引起广泛关注。然而，在当前的金属氧化物阴极中，可逆氧氧化还原仅能在高电压区域（通常超过3.5 V）实现。在此，我们在P2层状NaMg[FeMn□]O阴极材料中，于1.5 - 4.5 V的宽电压范围内实现了可逆氧氧化还原，其中本征空位位于过渡金属（TM）位点，而Mg离子位于Na位点。Na层中的Mg离子在电化学循环过程中，尤其是在高电压区域，充当“支柱”以稳定层状结构。TM层中的本征空位形成了“□ - O - □”“Na - O - □”和“Mg - O - □”的局部构型，从而在整个充放电电压范围内引发氧氧化还原。时间分辨技术表明，即使在10 C的条件下，P2相在1.5 - 4.5 V的宽电位窗口范围内仍能良好保持。研究发现，Mn离子和O离子的电荷补偿作用于整个1.5 - 4.5 V电压范围，而Fe离子的氧化还原仅作用于3.0 - 4.5 V的高电压区域。指向TM空位位点的O的非键2p轨道中的孤对电子是可逆氧氧化还原的原因，且Na位点中的Mg离子有效抑制了氧的释放。
